1. Field of the Invention
The present invention relates to a data recording/reproducing device which records data and reproduces data from a recording medium. The present invention also relates to a recording medium to be used in such a data recording/reproducing device.
2. Description of the Related Art
Recently, optical disk recording/reproducing devices have been receiving much attention for recording and reproducing large quantities of data. In order to achieve a higher memory density, various approaches have been taken from both a device standpoint and a recording medium standpoint. These approaches include optimization of data reproduction.
FIG. 11 is a block diagram showing an example of a conventional data recording/reproducing device. In FIG. 11, an optical disk 11 is supported by a driving shaft of a motor 12, thereby being rotated by the motor 12. An optical head 13 includes a semiconductor laser which emits a laser beam toward the optical disk 11, and a photodetector which photoelectrically converts incident light reflected off the optical disk 11 into a reproduced signal.
For recording a signal, a signal generator 14 externally receives data and modulates the received data by pulse position modulation (hereinafter, simply referred to as PPM) and/or pulse width modulation (hereinafter, simply referred to as PWM), thereby generating the signal that is to be recorded on the optical disk 11. The signal is then output to a laser driving circuit 15. The laser driving circuit 15 controls and drives the semiconductor laser of the optical head 13 in response to the input signal so that the semiconductor laser outputs an optical signal corresponding to that input signal. The optical head 13 is supported and moved by a moving mechanism (not shown) so as to trace a track(s) of the optical disk 11. When an optical signal is directed from the optical head 13 to the track(s) of the optical disk 11, while this tracing takes place, a signal corresponding to the optical signal is recorded on the track of the optical disk 11. The signal is recorded in a predetermined data unit, i.e., a sector.
On the other hand, the signal recorded on the optical disk 11 is reproduced as follows. While the optical head 13 traces the track(s) of the optical disk 11, light reflected off the optical disk 11 is received by the photodetector. Then, the photodetector photoelectrically converts the received signal into a reproduced signal. The reproduced signal output from the photodetector is input to a preamplifier 16 so as to be amplified. The resultant reproduced signal is input to an equalizer 17. The equalizer 17 compensates the frequency characteristics of the signal. Then, the resultant reproduced signal A (shown in FIG. 12A) output from the equalizer 17 proceeds to each of two subsequent processing sequences shown in FIG. 11. One of the sequences includes a differentiator 18 and a zero crossing comparator 19, for digitizing the reproduced signal A. The other one of the sequences includes a high-pass filter 21, an amplifier 22, a clamping circuit 23, an envelope detector 24 and a comparator 25, for forming a digitized gate signal in accordance with a level of the reproduced signal A.
The differentiator 18 differentiates the reproduced signal A output from the equalizer 17 and outputs a signal B shown in FIG. 12B to the zero crossing comparator 19. The zero crossing comparator 19 generates a pulse signal every time the signal B becomes zero. As a result, a digitized signal C shown in FIG. 12C is obtained In such a manner, the reproduced signal A reproduced from the optical disk 11 is digitized.
On the other hand, the high-pass filter 21 removes a low-frequency component of the reproduced signal A output from the equalizer 17. The resultant signal with the low-frequency component removed is amplified by the amplifier 22, and is given a constant clamp voltage Vo shown in FIG. 12D as a reference level of the signal. Accordingly, a signal D shown in FIG. 12D is obtained which is then output to the envelope detector 24 and the comparator 25. The envelope detector 24 has a predetermined time constant such that when the signal D output from the clamping circuit 23 is input thereto, the level of the signal D is clipped by approximately 40%, thereby forming a threshold signal Sh shown in FIG. 12D which is output to the comparator 25. The comparator 25 compares the signal D shown in FIG. 12D and the threshold signal Sh so as to output a gate signal E shown in FIG. 12E. The gate signal E represents periods where the reproduced signal A output from the optical disk 11 reaches a certain level.
An AND circuit 26 determines an AND signal of the digitized signal C and the gate signal E so as to form a signal F shown in FIG. 12F.
By digitizing the signal A reproduced from the optical disk 11 through the differentiator 18 and the zero crossing comparator 19 as set forth heretofore, the digitized signal C representing the correct timing can be obtained.
However, when the reproduced signal A contains noise N as shown in FIG. 12A, this noise N may be undesirably digitized as well. In order to solve this problem, the gate signal E that represents the periods where the reproduced signal A is reaching the certain level, is formed through the high-pass filter 21, the amplifier 22, the clamping circuit 23, the envelope detector 24 and the comparator 25, and the AND of the signal C and the gate signal E is determined so as to obtain the signal F which is exclusive of effect caused by the noise N.
The signal F is input to a phase look loop (PLL) circuit 27 so as to be synchronized with a clock signal before being input to a demodulator 28. The demodulator 28 demodulates the signal synchronized with the clock signal (i.e., the signal subjected to PPM and/or PWM) so as to form a data signal and an error correction code signal The data signal is corrected based on the error correction code signal and then the data signal is output.
The above-described process of reading signals from the optical disk 11 is performed in a predetermined data unit, i.e., a sector.
Moreover, in the process of reading the signals, an error detector 29 determines whether the data signal has an error or not, for example, by a parity check. When it is determined that the data signal has an error, the process of reading the signal is repeated. Such a repeating of the process is directed by a host processor (not shown) or the like which exercises general control over each of the blocks shown in FIG. 11, in accordance with the flow chart of FIG. 13.
As shown in FIG. 13, when an instruction for reading the data occurs (Step 101, Yes), a count number RC of a retry counter is initialized to 0 (Step 102). Then, a signal for one sector is reproduced from the optical disk 11. A data signal and an error correction code signal are formed from the thus-obtained reproduced signal as described above (Step 103). Thereafter, it is determined whether or not the data signal has an error (Step 104). If there is no error in the data signal (Step 104, No), the process of reading data for one sector is completed (Step 105).
If there is an error in the data signal (Step 104, Yes), the count number RC of the retry counter is incremented by 1 (Step 106) which is checked to ensure that a predetermined permissive value N (Step 107, No) has not been exceeded. Then, the process returns to Step 103, whereby a signal from the same sector is reproduced again from the optical disk 11 so as to form a data signal and an error correction code signal (Step 103). If there is no error in the data signal (Step 104, No), the process of reading data for one sector is completed (Step 105). If there is an error in the data signal (Step 104, Yes), Steps 106, 107, 103 and 104 are repeated again. Each of Steps 106, 107, 103 and 104 is performed until the count number RC of the retry counter exceeds the permissive value N. When the count number RC exceeds the permissive value N (Step 107, Yes), the process stops and the occurrence of an error is reported (Step 108).
Data can be rewritten in a portion in a recording region of the optical disk 11 for about 0.5 to 1 million times depending on the conditions of use of the optical disk 11. However, a memory film of this portion can easily deteriorate. When such a deterioration of the memory film occurs, the low-frequency component contained the reproduced signal increases.
For example, a reproduced signal A containing the low-frequency component may become as represented in FIG. 14A. In this case, even when the reproduced signal A is digitized by the differentiator 18 and the zero crossing comparator 19 at correct timing so as to obtain the digitized signal C shown in FIG. 14C, a gate signal E may be incorrectly formed as shown in FIG. 14E due to undesirable level variation caused by the low-frequency component contained in the reproduced signal A. In this case, when an AND of the digitized signal C and the gate signal E is determined as shown in FIG. 14E, one of the pulses of the digitized signal C is undesirably neglected, thereby causing an error.
If such an error caused by the low-frequency component is present in one sector, the occurrence of errors is unavoidable in that sector, even when the data in that sector is repeatedly read-out.